Signal amplitude limiting circuits



9 A. P. MONTGOMERY I 2,224,794

SIGNAL AMPLITUDE LIMITING CIRCUITS Filed May 13, 1939 Patented Dec. 10, 1940 PATENT. OFFICE 2,224,794 SIGNAL AMPLITUDE LIMITING CIRCUITS UNITED STATES Andrew P. Montgomery, Narbcrth, Pa., assignor,

mesne assignments,

to Philco Radio and Television Corporation, Philadelphia, Pa., a corporation of Delaware Application May 13, 1939, Serial No. 273,555

18 Claims.

over, without the use of expensive filter means.

but largely through the use of low cost diode elements. The invention is adapted for use in the reception of music, speech, facsimile, television, telegraph and control signals, or, in short, whereever noise impulses may be of greater amplitude and/or shorter duration than the useful modulation components of the desired signals.

An important object of this invention, therefore, is to provide novel and simple, but highly efiective, circuits for limiting the amplitude and steepness of wave front of interfering or undesired noise signals.

Another object of this invention is to provide means for the protection of various circuit elements or signal utilization means from the shock effects produced by atmospherics, man-made interferences and the like, and to accomplish this with the introduction of a minimum of additional parts.

Still another object of this invention is to provide a low cost electronic noise reducing system which exhibits highly advantageous frequency discriminatory characteristics of the low pass types heretofore unobtainable even with more costly filter systems.

The invention itself, as well as other objects thereof, may be clearly understood by reference to the accompanying drawing, the single figure of which is a diagrammatic illustration of one embodiment of the invention.

The figure broadly discloses a source of modulated carrier signals I, a coupling transformer 2, acarrier frequency amplifier 3, a second coupling transformer 4, and a signal utilization means comprising the ,circuits shown to the right of the terminals XY. Representative of the subject matter of the invention are the diode elements 5 and 6 and their associated circuits. As will be explained hereinafter, it is the cooperative action of these diode circuits which is responsible for the discriminatory action of the system.

Referring now in more detail to the drawing,

the carrier frequency amplifier 3 may be provided with an automatic gain control means comprising the shunt-connected capacitor I and resistor 8 as is well known in the art. If the amplifier tube 3 is of the screen grid type, the screen voltage may be derived from an appropriate point on the potentiometer comprising the resistors 9, l0, and II which are serially connected across the plate voltage supply. The output circuit for the amplifier 3 may comprise the tuned transformer 4 whose primary is connected between the anode of the said amplifier and a suitable source of potential, such as that provided by the voltage drop across resistors 9 and II]. In the preferred embodiment of theinvention, the transformer 4 is double tuned, of high Q, and about critically coupled, but the invention is not limited to this mode of operation.

To the output terminals X-Y of the transsignal utilization means. In the specific embodiment illustrated, there is shown a gas triode (or Thyratro'n) control circuit which is energized or triggered directly by the carrier frequency signal. It should be understood, however, that the invention is not so limited. Thus, to the terminals XY may be connected any system to which it is desired to supply a relatively noisefree carrier signal. Moreover, the signal source I may provide signals of any desired frequency such as, for example, signals of radioor intermediate-frequency. Accordingly, the circuits to the right of the terminals X-Y might be replaced by a first or second detector of a superheterodyne radio receiver, followed by the usual circuit components, as is well understood in the art.

The gas triode control circuit illustrated is one which is fully described and claimed in my copending application Serial No. 265,328, filed March 31, 1939. Briefly, the circuit comprises a gas triode l2 on whose grid and plate electrodes there is impressed an alternating current voltage derived, by way of the transformer l3, from a source [4, which may be a commercial 60 cycle, 110 volt source. By means of a phase shifting network l5-l6, and the transformer tap II, the phase and magnitudes of the plate and grid voltages are so adjusted that the gas triode will fire only in the presence of an input signal (in this case a modulated carrier signal) of a predetermined amplitude. The resistor-capacitor combination l8 provides a self bias for the gas triode. A load circuit l9, which may be 'a relay, may be provided as shown. An example of a specific use of such a gas triode, capable of being triggered by a modulated carrier wave, is fully disclosed in the copending application of E. 0. Thompson and David Grimes, Serial No. 220,366, filed July 20, 1938, which discloses a remote control system for a radio receiver. It has been found that, unless special precautions are taken, such gas triode circuits may be fired or triggered by the arrival of interference pulses, such as those generated by the opening or closing of electric power circuits, atmospherics, or the like. Accordingly, it is one of the objects of this invention to provide novel and effective means for discriminating against such interference pulses, and which means may be conveniently incorporated in the control signal transfer or amplifier circuits preceding the controlled circuits.

The amplitude limiting means, as disclosed in the specific embodiment illustrated, comprises two non-linear impedance elements, which may be diodes, 5 and 6, whose functions are mutually cooperative. The first of these non-linear impedances is the diode 5 which may be connected across the primary winding 20 of the transformer 4 as shown. This diode is preferably biased to such a degree that its anode remains at a negative potential with respect to its cathode for all signals not exceeding a predetermined amplitude. Normally this predetermined amplitude will be of the order of the amplitude of the desired signals at this point in the circuit. Thus, the diode 5 will normally conduct only when voltages exceeding this bias appear across the primary winding 20. When the diode is conducting, the primary oi the transformer 4 will be effectively short-circuited and hence the transfer of undesired, high amplitude signals will be prevented.

Thus the biased diode 5 constitutes a signalamplitud-e-responsive signal-limiting means, since it responds to the amplitude of the signals and limits the transferred signals to a predetermined amplitude.

The bias for the diode 5 may be supplied by means of a battery or by connecting the diode cathode to a point whose potential is higher than the potential of the primary winding 20. In the specific embodiment shown in the drawing, the cathode of diode 5 is connected to the junction of the resistor H and the point B+. The voltage drop across this resistor then forms the necessary bias supply for the diode 5. In order that the resistor I I may not add too great a resistance I in series with the diode it may be desirable to bleed a relatively large current through the potentiometer 9-I0-l I, thus providing the necessary bias across resistor ll while permitting its resistance to remain within reasonable limits. The resistor II is preferably by-passed for carrier frequency by a condenser 2|. Obviously if a battery is employed as a bias supply, the resistor ll may be eliminated.

In addition to the action above noted, it will be shown hereinafter that the action of the diode 5 is modified and enhanced by the action of the second diode element 6. This second diode, in series with the capacitor 22, is connected in shunt with the secondary winding 23 of the transformer 4. A resistor 24 may be connected in shunt with either the diode 6 or the capacitor 22 to provide a direct current leakage path for said capacitor. If now the tuned winding 23 provides a modulated carrier wave, and if the time constant RC of the delay circuit comprising the resistor-capacitor circuit 22-24 is comparable to the period of the highest useful harmonic of the modulation envelope, then the modulated signal will be transmitted through the system with substantially no loss. This action obtains because the condenser 22 acquires a charge which provides a potential across the resistor 24 which opposes the flow of current through the diode 6. Thus the diode 6 conducts only for a very short part of each cycle of the carrier, and hence only a negligible amount of energy is drawn from the transformer 4. Stated in another way, the loading effect of the diode 6 on the transformer is small. This action may be explained by noting that the time constant of' the circuit 22-24 has been made low enough to permit the voltage thereacross to follow substantially the useful excursions of the carrier envelope. If now there arrives a noise pulse of short duration whose effect is to produce momentarily a sharp change in envelope amplitude, the voltage across the circuit 22-24 will be unable to follow that excursion, and as a result the diode and .associated delay network constitute, for the length of the short pulse, a heavy load on the signal transfer circuit, or more specifically the transformer 4, whereby the latter is, for all practical purposes, completely short-circuited for that time. It is to be noted that this condition may obtain, at least partially, even though the noise pulse be of a lesser amplitude than the peak useful signal pulse.

Thus the diode 8 and the R-Ccircuit 22-24 constitute a signal-duration-responsive signallimiting means, since this combination responds to the duration of the signals and limits or discriminates against signals of short duration compared to that of the desired signals.

When the diode 6 becomes conductive in the presence of short noise pulses having steep wavefronts, as explained above, the effect on the tuned secondary of the transformer 4, is that of a low resistance across its terminals X-Y, which is, of course, equivalent to the addition of a high series resistance in the resonant circuit 23-25. Thus the action of the diode 6 is to reduce greatly the Q of the resonant secondary circuit in the presence of noise signals, and hence also the overall gain of the system. On the other hand, the reduction in secondary circuit Q tends to decrease the loading effect of the secondary on the primary, and accordingly the reduction in secondary circuit Q is accompanied by an increase in the effective Q of the primary tuned circuit. Now since the impedance presented to the plate circuit of the amplifier 3 by the resonant primary circuit is directly proportional to the Q of the latter circuit, it will be seen that where the amplifier tube 3 has a high impedance, as for example when a pentode is employed, the voltage developed across the primary of the transformer 4 will be approximately directly proportional to the parallel impedance of the tuned circuit 20-26, .and hence to its effective Q. Accordingly, it is seen that the action of the diode 6 in placing a low resistance across the secondary of transformer 4 in the presence of undesired noise pulses, actually tends to increase the voltage across the primary winding 20. This increase in primary voltage would tend partially to oppose the gain reducing effects of the diode 6 were it not for diode 5, and this undesirable tendency would be especially pronounced where a diode with a very low minimum resistance is not available. It is, therefore, an important part of this invention to provide the biased diode 5 across the primary winding of the transformer 4, as hereinbefore described, for the purpose of preventing or squelching the above-mentioned increase in primary voltage which tends to accompany the action of the diode 6.

Thus, the desired modulated carrier wave, whose envelope presents no steep wave fronts, or changes in amplitude, will pass through the system and arrive at the terminal X-Y without appreciable attenuation, while noise pulses, occurring separately or accompanying the desired signal, will be greatly attenuated by the combined and mutually cooperative action of the diodes 5 and 6. It'may be observed that where the noise pulses exceed the amplitude of the desired signals, the diode 5 would be able to act independently of thediode 8 to squelch the said noise pulses. Nevertheless, the accompanying effect of thediode 6 in increasing the primary voltage will enable a much larger proportion of the noise pulse energy to be dissipated on the primary side of the transformer than would otherwise be the case. Moreover, the presence of the diode 6 will enable the diode 5 to act on noise pulses having an amplitude which would .normally not be of sufficient magnitude to exceed the delay bias of the diode 5. This, of course, is due to the action of the diode 6 which tends to increase the magnitude of the noise voltage across the winding 20. Thus it will be observed that, in accordance with this invention, it is entirely possible for both diodes to act on noise pulses of steep wavefront having amplitudes less than that of the desired signal amplitude, while having little or no effect on the desired signals.

No claim is made herein to the diode 6 and its circuit per se, this being disclosed and claimed in an earlier filed copending application of R. L. Campbell Serial No. 269,494, filed August 22, 1939, but the present invention comprises this feature in cooperative combination with the diode 5 and associated elements, as above described.

While it has been stated that in the preferred form of the invention the coupling means 4 may be a double tuned and approximately critically coupled transformer, it should be understood that coupling means other than transformers may also be employed. Thus any coupling system might be employed which is characterized in that the short-circuiting of the output terminals thereof produces an increase in the input impedance of the coupling system. For example, the transformer 4 might be replaced by a quarter wave-length transmission line, or impedance inverter, terminated for example in its characteristic impedance. It is well known in the art that when the load end of such a line is short-circuited, the line will appear, from its input end, as a parallel resonant circuit having a high impedance. Such a coupling means could be employed in certain high frequency applications of the present invention.

In an application of the circuits of this invention to a remote control signal amplifier employing a gas triode operated relay, as described in the above mentioned application of E. 0. Thompson and David Grimes, the resulting discrimination against a large variety of noise pulses was highly satisfactory. In this applicatiomthe control signals consisted of pulses obtained by keying a source of signals of radio frequency, so as to give a constant amplitude wave train of the order of 0.05 second duration followed by a space of similar duration, and so on. Obviously, where such very slow signals are employed, the time constant of the resistor-capacitor circuit 22-24 may :be made very large, and in the device referred to, the resistor 24 was 750,000 ohms, while the capacitor 22 was 0.02 microfarad, giving a time constant RC of 0.015 second. It is also obvious that where it is desired to receive voice, music, or high modulation frequencies it would be necessary to make the time constant RC correspondingly smaller in order to prevent the attenuation of the higher modulation frequency components.

In the particular device referred to above, the two diodes 5 and 6 were combined in a single envelope, a type 6I-I6G double diode being employed. The amplifier tube 3 was a type GJ'IG pentode having a plate resistance of well over a megohm. The potentiometer resistors 9, l0, and H were of 8500, 50,000, and 2000 ohms respectively, and the plate voltage supply 3+ was 250 volts. Capacitors 1 and 2| were of 0.01 and 0.05

microfarad respectively, while the resistor 8 had a resistance of two megohms.

The values,.of the circuit elements here given are in no way intended to limit the scope of this invention, since it is obvious that these values will vary greatly depending upon the specific use to which the circuits are put. Moreover, although the invention has been described with particular reference to the drawing, it will be understood that the invention is capable of various forms of physical expression, and is to be limited only by the scope of the appended claims.

I claim:

1. A signal amplitude limiting system comprising, in combination with a vacuum tube coupling means having a primary and a secondary winding, a diode and capacitor in series shunted across said secondary winding, a resistor connected in shunt with one of said serially-connected elements, and a bias voltage source and a second diode serially connected across said primary winding. I

2. A signal amplitude limiting. system comprising, in combination with a vacuum tube coupling means having a tuned primary winding and a tuned secondary winding, the coupling between said windings being approximately critical, a diode and capacitor, in series, shunted across said secondary winding, a resistor connected in shunt with one of said serially-connected elements, and a bias voltage source and a second diode serially connected across said primary winding.

3. A signal amplitude limiting system comprising a source of signals, a vacuum tube amplifier for amplifying said signals, a signal utilization means, a coupling means having a primary and a secondary winding for coupling the output of said amplifier to said utilization means, a diode-capacitor combination connected 1 in series, connections for shunting said combination across said secondary winding, a resistor connected in shunt with one of the elements in said combination, and a bias voltage source and a second diode serially connected across said primary winding.

4. A signal amplitude limiting system comprising a source of signals, a vacuum tube amplifier for amplifying said signals, said vacuum tube amplifier having a high plate resistance, a signal utilization means, a double tuned transformer for coupling the'output of said amplifier to said utilization means, said transformer having primary and secondary windings, a diodecapacitor combination connected in series, connections for shunting said combination across said secondary winding, a resistor connected in shunt with one of the elements in said combination, and a bias voltage source and a second diode serially connected across said primary winding.

5. A signal amplitude limiting system comprising a source of signals, a vacuum tube am- .said series-connected elements across said resonant secondary winding, means for providing a high resistance direct current path for the rectification products of said diode, a second diode connected imshunt with said resonant primary winding, and means for applying a bias voltage to said second diode whereby said second diode.

becomes conductive only to signal voltages exceeding a predetermined amplitude.

6. A system for limiting the amplitude or noise signals having steep wavefronts, comprising a source of signals, a vacuum tube amplifier for amplifying said signals, means for coupling the output of said amplifier to a signal utilization means, said coupling means having input and output terminals, a non-linear impedance means and a capacitor connected in series, connections for shunting said series-connected elements across said output terminals, a resistor connected in shunt with one of said series elements, and a bias voltage source and second non-linear impedance means serially connected across said input terminals.

7. A system for limiting the amplitude of noise signals having steep wavefronts, comprising a source of signals, a vacuum-tube amplifier for amplifying said signals, means for coupling the output of said amplifier to a signal utilization means, said coupling means having input and output terminals and characterized in that shunting of said output terminals with a relatively low impedance results in an increased impedance between said input terminals, a nonlinear impedance means and a capacitor connected in series, connections for shunting said series-connected elements across said output terminals, a resistor connected in shunt with one of said series elements, and a bias voltage source and second non-linear impedance means serially connected across said input terminals.

8. A system for limiting the amplitude of noise signals having steep Wavefronts, comprising a source of signals, a vacuum tube amplifier for amplifying said signals, means for coupling the output of said amplifier to a signal utilization means, said coupling means having input and output terminals and characterized in that shunting of said output terminals with a relatively low impedance results in an increased impedance between said input terminals, said coupling means being further characterized in having a high Q, a diode and a capacitor connected in series, connections for shunting said seriesconnected elements across said output terminals, means for providing a high resistance direct current leakage path for the rectification products of said diode, a second diode connected in shunt with said input terminals, and means for applying a bias voltage to said second diode whereby saidsecond diode becomes conductive only to signal voltages exceeding a predetermined amplitude.

9. A system for attenuating noise signals, comprising a source of desired modulated carrier frequency signals, means for coupling said source to a signal utilization means, said coupling means including a. double tuned transformer having primary and secondary windings, a first non-linear impedance means in shunt with said secondary winding, time delay means operatively associated with said non-linear impedance means for rendering the latter substantially non-conductive to desired signals and conductive to noise signals having a time duration substantially less than a predetermined maximum, whereby such noise signals are attenuated while said desired signals are transmitted without substantial loss, and a bias voltage source and a second non-linear impedance means serially connected across said primary winding to augment the noise reducing action of said first non-linear impedance means.

10. A system for attenuating noise signals, comprising a source of desired modulated carrier frequency signals, means for coupling said source to a signal utilization means, said coupling means having input and output terminals and characterized in that short circuiting of said output terminals results in an increased impedance between said input terminals, a first non-linear impedance means in shunt with said output terminals, time delay means operatively associated with said non-linear impedance means for rendering the latter substantially non-conductive to desired signals and conductive to noise signals having a time duration substantially less than a predetermined maximum, whereby such noise signals are attenuated while said desired signals are transmitted without substantial loss, and a bias voltage source and a second non-linear im pedance means serially connected across said input terminals to augment the noise reducing action of said first non-linear impedance means.

11. A system for attenuating noise signals, comprising a source of desired modulated carrier frequency signals, means for coupling said source to a signal utilization means, said coupling means having input and output terminals and characterized in that short circuiting. of said output terminals results in an increased impedance between said input terminals, a first diode element, a capacitance element serially connected with said diode element, said serially-connected elements being in shunt relation with said output terminals, a resistor effectively in shunt with said capacitance element, said capacitance element and said resistor together having a time constant which is large compared to the duration of individual noise pulses, a second diode element connected in shunt with said input terminals, and means for applying a bias voltage to the terminals of said second diode whereby said second diode becomes conductive only when the voltage across said input terminals exceeds said bias voltage.

12. In a signalling system, a high impedance source of carrier signals modulated at a relatively low rate, which signals may be accompanied by noise signals of relatively steep wave front and short duration tending further to modulate said modulated carrier but at a higher rate; a signal utilization means, means for coupling said source and said utilization means including a double tuned and approximately critically coupled transformer having primary and secondary windings, means providing a shunt path across said secondary winding, means comprising a time delay circuit for rendering said path substantially non-conductive for desired signals and conductive for noise signals, the primary impedance of said transformer, and hence the voltage thereacross, increasing when said shunt path is conductive, means providing a shunt path across said primary winding, said last-named shunt path being normally non-conductive, and means for rendering said last-named shunt path conductive when the voltage across said primary winding exceeds a predetermined amplitude.

13. In a signalling system, a high impedance source of carrier signals modulated at a relatively low rate, which signals may be accompanied by noise signals of relatively steep wave front and short duration tending further to modulate said modulated carrier but at a higher rate, a signal utilization means, means for coupling said source and said utilization means including a double tuned and approximately critically coupled transformer having primary and secondary windings, a diode and capacitor in series shunted across said secondary winding, a resistor connected in shunt with one of said serially connected elements, said diode tending to act as a short circuit on said secondary winding in the presence of noise signals, whereby the primary impedance of said transformer, and hence the voltage thereacross, increases when said diode acts to short circuit said secondary winding, a second diode connected in shunt with said primary winding, and means for applying a bias voltage to said second diode whereby said second diode becomes conductive only to voltages exceeding the normal carrier signal voltage across said primary winding.

14. A noise signal reducing system, comprising a signal transfer device having input and output terminals [and characterized in that shunting of said output terminals with a low impedance causes an increase in voltage across said input terminals, means providing a shunt path across said output terminals, means for causing said shunt pathto present high impedance to desired signals and low impedance to noise signals, whereby the voltage across said input terminals increases whenever noise signals are present, means providing a shunt path across said input terminals, and means for rendering said last-mentioned path non-conductive when the voltage across said input terminals does not exceed a predetermined a signal-amplitude-responsive level and for rendering the said path conductive when the said voltage exceeds said level.

15. In a signal system including a coupling means having input and output terminals and characterized in that short-circuiting of the output terminals causes an increase in the impedance between said input terminals, the provision of a signal-amplitude-responsive signal-limiting means coupled to said input terminals, and a signal-duration-responsive signal-limiting means coupled to said output terminals.

16. A system for attenuating undesired signals, comprising a source of desired modulated carrier frequency signals, said source having a relatively high internal impedance, means for coupling said source to a signal utilization means, said coupling means having input and output terminals and characterized in that a reduction of impedance across said output terminals results in an increased impedance between said input terminals, signal-limiting means coupled to said input terminals, and a signal-duration-responsive signal-limiting means coupled to said output terminals.

17. In a signal transfer network having a resonant input circuit and a resonant output circuit and characterized in that a change in the Q of one of said circuits produces an opposite change in the Q of the other of said circuits, the provision of a signal-amplitude responsive signal-limiting means coupled to one of said circuits, and a signal-duration-responsive signal-limiting means coupled to the other of said circuits.

18. A signal amplitude limiting system comprising a source of signals, a vacuum tube amplifier for amplifying said signals, said vacuum tube amplifier having a high plate resistance, automatic gain control means connected to said amplifier for automatically adjusting the gain of said amplifier in inverse proportion to the amplitude of the signals from said source, a signal utilization means, a double tuned transformer for coupling the output of said amplifier to said utilization means, said transformer having primary and secondary windings, a diode-capacitor combination connected in series, connections for shunting said combination across said secondary winding, a resistor connected in' shunt with one of the elements in said combination, and a bias voltage source and a second diode serially connected across said primary winding.

ANDREW P. MONTGOMERY. 

